As used herein, the term artificial ground water is used for infiltrated surface water, and by the term raw water is meant ground water, artificial ground water, or surface water.
In many places, potable water requiring treatment and purification for removal of iron and manganese is treated in a water treatment plant by adding oxygen to the water. This leads to precipitation of impurities, which are filtered out to leave purified water. Oxygen is typically added to the water in the form of pure oxygen, or by aeration of the water through the use of a cascade aerator. Because the concentration of oxygen necessary to precipitate metal ions from solution is comparatively low, iron and manganese are precipitated through the filtration process described above, wherein the filter encompassed one or more layers of sand through which the water is passed. Capital investment and operating costs for such a purification plant are high since the sand layers which function as the filter material must be regenerated from time to time to enhance the removal of the precipitated metal ions of manganese and iron.
Alternatively, ground water has been purified in situ as described in Swedish Patent Specification No. 6903544-2. Water containing oxygen or oxygen-releasing substances was introduced intermittently into an aquifer through a number of satellite wells or satellite pipes, arranged vertically at a predetermined distance and surrounding an extraction well or extraction pipe.
As used herein, the term “satellite wells” is used to indicate any means arranged vertically in an aquifer by which treated water may be introduced into a filter, basin, aquifer or similar purification or filtration system through which such water is to be purified. However, it is to be understood that “satellite wells” are also used to withdraw water for introduction into other satellite wells. Representative examples of satellite wells include wells, pipes, hoses, tubes and similar devices.
Oxygenated water introduced into an aquifer creates a suitable environment for the growth of certain microbes enhancing chemical and metabolic adsorption, oxidation and precipitation of compounds in the existing ground layers, thereby utilizing the ground layers as a reaction and filtration medium. The microbes employed are often organisms being active primarily in zones created between oxidizing and reducing conditions. Water containing oxygen or oxygen-releasing substances was added intermittently over a period of approximately twenty-four (24) hours. During the addition of oxygenated water or water containing oxygen-releasing substances, the extraction well could not be used as a purified water source since concurrent use of the extraction well and the satellite wells would cause the failure of the filtration mechanism or a disturbance in its operation, resulting in a deterioration of the quality of water. To extract water on a continuous basis, two or more extraction wells or suitable means for storage has been required to circumvent the above-described disadvantages, enabling the withdrawal of purified water to be alternated between at least two extraction wells or between the well and the storage means. Regeneration was performed on the dormant extraction well or wells if necessary during withdrawal of purified water from the alternate extraction source.
To eliminate the drawback with a discontinuous extraction of water another alternative of ground water purification in situ has been described in Swedish Patent Specification No. 8206393-4. An advantage of that invention is the elimination of the need for two or more extraction wells, permitting purified water to be pumped continuously from a single extraction well.
During the operation oxygenated/conditioned water is fed to at least one but less than all of the satellite wells, and simultaneously with such introduction, water is drawn from at least one satellite well for the purpose of creating a purification zone for the adsorption, transformation, oxidation, precipitation or volatilization of contaminants. In particular, water conditioned with oxidizing agents or other beneficial additives is utilized to reduce the concentration of impurities from untreated water, rendering the water “purified.” The water fed to the satellite wells may consist of conditioned water drawn from at least one other satellite well and if desired also a portion of the purified water drawn from the extraction means.
By changing the plurality of satellite wells into which conditioned water is introduced, and by alternating the satellite wells from which water is withdrawn, the user optimizes the purification zone in which contaminants are precipitated. Any number of satellite wells greater than two may be used in this system, depending upon the size of the purification system, hydrogeologic conditions, biogeochemical conditions, the concentration of impurities contained in the water, etc. Conditioned water may be introduced into the satellite wells for a predetermined period, ranging from a few hours to a few days or longer.
The process according to Swedish Patent Specification No. 8206393-4 also enables the continuous filtration and purification of raw water utilizing a basin, wherein a filter comprised of naturally occurring or man-made material is contained and utilized. For example, soil may be excavated to form a basin-shaped cavity with a mantle surface, and a sealing layer of clay, concrete, plastic, water-resistant cloth or the like may be applied to the mantle surface to form an inner wall. Satellite wells for the introduction of conditioned water, are arranged vertically inside the mantle within the basin, and the basin is filled with a filter material, such as sand. The extraction means may be located in the centre of the basin; the satellite wells for introducing treated water are arranged between the mantle and the basin centre, around the interior of the periphery of the inner wall of the basin, so as to enable the introduction of conditioned water into the purification system through a plurality of the satellite wells, extraction of water through at least one satellite well and the continuous withdrawal of purified water through the extraction means. In the man-made aquifer, water is intermittently drawn from at least one satellite well, is conditioned and is further introduced into at least one other satellite well. Alternating the satellite wells used for recharge with the satellite wells used for withdrawal of water in this manner effectively alternates the purification zones utilized for the precipitation of contaminants.
WO02/48469 describes means and methods for purification of water, where these also can be used in an artificial aquifer, e.g. a basin filled with sand or similar material. In the description, the tightening cloth against the ambient soil is said to be arranged below the soil surface, which means that the top part is open against the atmosphere. The same arrangement is also true for the Swedish Patent Specification No. 8206393-4. This will give rise to an overflow and out wash of material from the basin because of the high water pressures that are built up in the basin from the infiltration of raw water and recharge into the satellite wells.
EP 04749194.9 describes a method for decreasing the contents of metals, metalloids, nitrate and nitrite, respectively, in raw water flowing through a basin containing naturally occurring filter material, whereby in case of decreasing the contents of metals or metalloids water containing oxygen or oxygen-releasing substances and/or metabolic reactions of naturally occurring micro-organisms and in the case of decreasing the contents of nitrate and nitrite water containing denitrification organisms and/or substrates therefore is intermittently introduced to the basin through a number of satellite wells arranged around at least one extraction well for purified water. The method is characterized in that said water containing oxygen or oxygen-releasing substances and/or metabolic reactions of naturally occurring micro-organisms or denitrification organisms and/or substrates therefore is homogeneously distributed by satellite wells arranged vertically in said basin, and the water to be purified is fed into the basin by horizontally arranged feeding pipes and that the extraction of purified water from said at least one extraction well is such that a substantial negative pressure is created around said at least one extraction well.
Moreover, according to said document the basin is hermetically sealed by a material impermeable to air and water and that all inlets and outlets through the material are impermeable to air and water.
When water is purified by passing through a reaction zone created by a plurality of satellite wells arranged vertically and surrounding an extraction well, the flow pattern from the satellite wells of injected water becomes generally parallel to the flow pattern of the water to be purified. When this generally parallel flow pattern is established, a discontinuous radial purification pattern develops between the satellite wells and between the satellite wells and the extraction well, resulting in that the raw water being poorly purified. This gradually reduces the operability of the extraction well due to precipitation of iron and manganese, which occurs in the aquifer within close proximity to the extraction well.